Ocean survival possible 'under three days'

Someone who survived a fall from a cruise ship at night would quickly become exhausted and cold, battered by waves they cannot see, an expert says.

Survival medicine specialist Dr Paul Luckin says fatigue would be the biggest challenge for a young couple who went overboard almost two days ago if they weren't injured by the impact of hitting the water.

Paul Rossington and Kristen Schroder were caught on CCTV falling from the cruise liner Carnival Spirit, around 8.50pm (AEST) on Wednesday, off the NSW mid-north coast.

"They would be extremely fatigued and fatiguing very fast," Dr Lukin, an adviser to the Australian Maritime Safety Authority (AMSA) and police rescue teams around the nation, told AAP.

"Being in the water at night is particularly difficult because you can't see waves coming.

"If you're stuck in the water and constantly being hit in the face by water, that is tremendously fatiguing."

Swallowing sea water restricts the airway and causes vomiting and diarrhoea, adding to dehydration.